I have a creepy feeling that the answers to this are going to lean towards 'personal preference' but I want to know what other people think since this is a new(ish) thing for me.

In the past, being fixed ot the bed has always been a simple case of the padded leather cuffs being used to stretch the arms out to the top corners and ankles held together and pulled as far down as possible in the middle at the bottom. So I'm in a kind of "Y" shape.

That's about to change. We now have some proper bed restraints that will secure ankles, upper legs, wrists, Waist, chest and upper arms.

My question is... Is it best to go with the traditional 'Arms Down' position or is there something to be said for 'Arms Up'?

In picturs on sites like Humane Restraint, you see pics of people with their arms secured down neaxt to the waist.

But I've seen some instances (mainly in silly Tv shows or films, or in Fetish galleries) of people with their wrists secured up at neck level.

You didn't mention how each position feels to you. Since you are horizontal, the position won't matter to much unless you hands are directly over your head. At the positions you asked about, there's no discernible difference.
I would guess that hands down would last longer since that's the natural position.

Shackling the arms to the neck is a real posture. The first scene I ever did with my bondage mentor involved putting the sub in that position. It creates an awkward posture that emphasizes humiliation as well as immobility. So if you want to simply be immobilized, I would go with hands out. If you want to feel some humiliation on top of that, I could go with hands to the neck. Obviously there are far more humiliating postures you might use than hands to the neck, but that's my take on it.

Hands out creates fewer issues for circulation and joint immobility than hands to the neck, so if you're doing hands to the neck, I would end the scene (or at least that posture) somewhat sooner.

"We hurt the ones we love the most. It's a subtle form of compliment."

I have had my hands cuffed and connected to my collar with a carabiner. It immobilized me quite well. I don't know if it's just me, but it put my arms to sleep pretty quickly. My shoulders were bent too far and cut off circulation a bit. I would suggest trying the positions to see what is more comfortable, or how long it is comfortable for. I find the same thing happens when they're directly behind my head (making my whole body straight, I-shaped), or off to the sides, but it takes a little longer.

Thanks for the replies. I spent a while the other night in the arms down position and it was glorius. I know some people spend large sections of their life like it and for them it probably isn't that great, but as far as I'm concerned I could do it every day.

We have sufficient restrint straps to haver a set for waist and wrists and another up top for when I want to try the arms up. So I won't have to do major hardware adjustments each time I want to switch it over.

Judging by the replies here, it looks like my long stints in the straps will be arms down, but some time will spent arms up to make myself more accessible for sexual encounters.

I'm still curious though with regards to the arms up position in use in medical facilities. Especially since it has been described as 'Awkward' and 'Emphasising humiliation'. I'd have thought they would be the two things to avoid with the mentally ill...